It said the glitch is due to the compression of audio files, MS is working on a solution, but I am not sure if the patch is out yet. I suggest you switch to a PC and try it out again.
It has been a known issue for PPT 2016 and MAC, I have faculty complained about the same problem in multiple times, the temp solution is bypassing the built-in converter in PPT and use a third party one, such as this one: Wondershare PPT2Video.
